This invention relates to the coating of linear material such as metallic sheet, strip, strand, and especially wire, with metallic coatings in a hot dipped coating bath. More particularly the invention relates to the use of protective atmospheres and gas wiping of hot dipped coatings of aluminum-zinc on linear material and particularly wire and the like.
Metallic linear material such as sheets, strip and wire has been economically coated for many years by passing the linear material through a bath of molten metal such as molten zinc or aluminum. Usually the linear material has been a ferrous material such as steel or the like. The resulting outer coating of aluminum or zinc or sometimes other metals or alloys such as tin or terne (an alloy of lead with up to 25% tin) provides corrosion resistance to the underlying ferrous metal.
Linear material passing from a molten metal coating bath usually does not have a satisfactory layer of molten coating metal on its surface. The molten metal coating is invariably either too thick, too uneven, or both, or has some other defect which would prevent the molten metal from solidifying into a satisfactory metal coating upon the substrate metal. As a consequence, it has been customary to wipe the coating in some manner after the linear material leaves the molten coating bath in order to smooth and/or reduce the weight of the coating. Various wiping devices have been used to wipe the coating while it is still molten, including soft wipers such as asbestos and the like, rigid wipers such as rolls and scrapers, and occasionally layers of other materials through which the coated linear material passes. More recently gas wipers, or gas doctors, have been used to forcibly blow a gas such as air, steam or some inert or reducing gas against the molten coated surface of the linear material to remove excess molten metal and smooth the coating of molten metal.
In order to attain good adherence of the coating metal to the substrate metal it is necessary for the surface of the substrate to be clean prior to passage through the molten coating bath. The linear material must therefore be cleaned prior to being coated in order to provide a suitable clean, active substrate surface for contact with the molten coating bath. Once the substrate metal is clean it must be kept clean and active, i.e. oxide free, until it is submerged in the molten coating bath. It is therefore necessary to protect the substrate metal after cleaning either with a coating of flux or else by immersion in an inert or reducing atmosphere. Thus, ferrous linear material frequently enters the molten coating bath in a protective or oxygen excluding atmosphere. The protective atmosphere is composed of either an effectively inert gas or a reducing gas or gases. Inert or reducing atmospheres have also been used to protect the linear material as it exits from the molten bath to prevent detrimental oxidation of the surface of the coating while it is still hot both before and after the coating solidifies.
The protective atmosphere is usually contained in a hood which extends to or into the surface of the molten bath.
With the more frequent use of gas wipers for smoothing and wiping the molten coating, the use of an inert or a reducing gas to wipe the surface of the linear material has sometimes been adopted to prevent surface oxidation. In some installations, and particularly in wire wiping installations, the wiper has been enclosed in or attached to a chamber containing a protective atmosphere so that the molten coating on the wire is completely protected from exposure to the normal atmosphere until it is wiped. Such enclosed gas wiping operations have been more frequently used during the coating of linear material such as wire, rather than when coating larger material having extended transverse dimensions such as sheet or strip because of the difficulty in completely enclosing such larger material and also because the coating of wire tends to be more critical and "touchy" than the coating of sheet and strip. However, there is no overriding reason why wiping enclosures cannot be effectively applied to the coating of sheet and strip as well and some specialized installations have included this refinement.
The use of a non-oxidizing gas as both a wiping and a protective gas has been found to be particularly desirable in the wiping of wire material. Otherwise oxidized coating particles on the molten coating surface tend to increase the viscosity of the molten metal and result in buildup of a thick viscous oxide material layer which seriously interferes with effective gas wiping. The small circumference of the wire allows viscous rings of oxide material to form about the wire and break through the gas barrier resulting in thick rings of coating on the wire. Such coatings after solidification cracks and flake when the wire is bent.
Within the last decade a completely new coating has made its appearance on the market. This coating is composed of an alloy of aluminum and zinc, usually having a composition within a range of about 25 to 70% aluminum. The coating is usually a multi-phase coating having zinc-rich and aluminum-rich regions in the coating overlay and when formed from a hot dip coating, a thin intermetallic alloy layer between the overlay and the base metal. These multi-phase coatings have proven to have superior corrosion resistance and to be both economical and convenient to apply with the use of proper techniques by hot dip coating.
While aluminum-zinc coatings have proven to be very corrosion resistant and otherwise advantageous and to a large extent ordinary hot dipped coating apparatus has been found to be effective in the forming of the new coatings, some special problems have arisen in the production of such coatings and have been solved by new techniques, several of which are the subject of issued patents.
One problem which has arisen in the coating of wire in particular with aluminum-zinc coatings is the occurrence of small discolored depressions or craters in the surface of the final coating. These depressions look like actual bare spots or pin holes through the coatings, but when examined with a microscope prove to be only depressions. Nevertheless, because the coating at the bottom of the depressions is thinner than the surrounding coating and thus more subject to perforation by corrosion, and because the depressions have a burned appearance, which may be considered by many to be a blemish, such depressions or craters are undesirable. Because of their burned look these depressions have been called "powder burns". This type of blemish appears to be more or less unique to aluminum-zinc coatings. Similar blemishes are not found on galvanized or aluminized products. The defect has appeared usually and most noticeably upon hot dip coated aluminum-zinc coated wire which has been wiped by an inert or reducing gas wiper connected with a hood extending to the bath surface to prevent oxidation of the bath surface. Such an arrangement has been used to avoid the occurrence of oxide inclusions in the surface of the coating and has been successfully used in coating with other coating metals by prior workers.